Reaction Steps
To a solution of (6R,6aS,14aR)-1,6,8,14a-tetrahydroxy-11-((3R,4R,5R,6S)-4-hydroxy-3,5-dimethoxy-6-methyltetrahydro-2H-pyran-2-ylamino)-6a-methoxy-3-methyl-7,9,12,14-tetraoxo-5,6,6a,7,9,12,14,14a-octahydrobenzo[a]tetracene-2-carboxylic acid (50 mg, 0.073 mmol) in THF (5 mL) was added 2-methylethylamine (12 μL, 0.15 mmol), polystyrene carbodiimide (130 mg, 0.147 mmol, 1.12 mmol/g), and 1-hydroxybenzotriazole hydrate (20 mg, 0.15 mmol). The reaction mixture was stirred at room temperature under nitrogen for 12 h. The reaction was quenched by adding 1N HCl solution (5 mL) and brine (25 mL). The aqueous layer was extracted with methylene chloride (2×50 mL). The organic layer was separated, dried (MgSO4), filtered and concentrated to give the crude product. The crude material was purified by preparative HPLC (10:90 acetonitrile/water to 60:40 acetonitrile/water with 0.05% TFA over 10 min, then isocratic) and lyophilized to afford (6R,6aS,14aR)-1,6,8,14a-tetrahydroxy-11-((3R,4R,5R,6S)-4-hydroxy-3,5-dimethoxy-6-methyltetrahydro-2H-pyran-2-ylamino)-N-isopropyl-6a-methoxy-3-methyl-7,9,12,14-tetraoxo-5,6,6a,7,9,12,14,14a-octahydrobenzo[a]tetracene-2-carboxamide (8 mg, 15%) as a red-brown powder and as a mixture of diastereomers: 1H NMR (500 MHz, CD3OD) δ 8.11 (s, 1H) 6.59 (s, 1H), 6.19 (s, 0.2H), 5.95 (s, 0.8H), 4.97-4.89 (m, 2H), 4.10-4.03 (m, 1H), 3.75 (s, 3H), 3.73-3.69 (m, 1H), 3.64 (s, 2H), 3.59 (s, 3H), 3.57 (d, J=16.0 Hz, 3H), 3.47-3.35 (m, 1H), 3.23 (d, J=18.7 Hz, 1H), 3.09-3.02 (m, 1H), 2.22 (s, 3H), 1.27 (d, J=6.0 Hz, 2.4H), 1.23 (d, J=6.0 Hz, 0.6H), 1.14 (d, J=17.5 Hz, 6H); MS (ESI−) m/z 723 (M−H); HPLC >99% (AUC), tR 12.93 min.
